The present invention relates to a bicycle sprocket wheel, and more particularly to an improvement in a rear sprocket wheel which is a basic element of a multi-speed free wheel assembly mounted on a rear wheel hub of a bicycle for facilitating speed change.
The multi-speed free wheel assembly of a bicycle generally has a plurality of rear sprocket wheels which are different in diameter as well as in number of teeth and are coaxially mounted in order in parallel to one another on a rear wheel hub. The rear sprocket wheels cooperate for speed change with a derailleur (chain shifting device) and an endless driving chain which is stretched, through said derailleur, between a rear sprocket wheel and a front sprocket wheel connected to a driving crank. The driving chain is adapted to engageably shift from one sprocket wheel onto another when the derailleur is controlled, whereby a desired speed change is carried out.
In this speed change operation, it is most important that the running chain can be smoothly and quickly shifted to a selected gear position without disengagement from the free wheel assembly. In order to acquire this function, it has heretofore been proposed to tip the teeth of the sprocket wheel rather sharply or to reduce the thickness of each sprocket wheel. It has also been proposed to minimize the distance between adjacent sprocket wheels for easier shifting of the chain. These heretofore proposed prior arts solutions have proved to be effective to some extent, however, the results have not been entirely satisfactory. Thus, unexpected chain disengagement during gear change continues to trouble the cyclist almost as much as ever.
As well known in the bicycle manufacturing field, the gear change is performed in the manner as follows: Upon the derailleur being swung in one selected direction, a part of the chain which is passing through the derailleur is also swung together, so that the part which has been swung may be shifted from an initial sprocket wheel onto a desired one, that is, positioned in the same direction as said derailleur is swung. Consequently, such chain part, running ahead of said swung part, is forced to leave the initial sprocket wheel by the pulling action of the aforementioned chain part that has just been swung onto the desired sprocket wheel by means of said derailleur, and starts to gradually, engageably ride onto the latter as the initial and the desired sprocket wheels coaxially rotate. At this stage, it should be noted that the running chain will not be allowed to easily mesh with the desired sprocket wheel unless the chain is maintained tense during the shifting operation. In other words, as long as the chain is maintained in tension, it can be pulled up onto the desired sprocket wheel at any rate as the same rotates.
In the course of shifting, the chain frequently collides with the side wall of the first accessible tooth that it comes across and is thereby prevented from catching said tooth firmly. If the chain fails to catch the said first tooth, the second or the third teeth that follow in order must obviously be caught instead of said first tooth, otherwise the chain tension will be excessively loosened and finally the chain will fall off the sprocket wheel.
In the conventional sprocket wheel, however, once the chain fails to catch the first tooth, the second or the third teeth cannot be easily caught because each of these teeth is identically formed and arranged into line along the circumference of the sprocket wheel so that the side wall of these teeth all lie in the same plane. Affordingly there is no engageable means for the chain to catch the teeth. As a result, the chain is required to run skiddingly along a series of aligned side walls of the sprocket teeth without developing a driving force.
The fact is also well known in the bicycle field that the running chain is liable to fail to mesh with the sprocket teeth, especially in the case where it shifts from a smaller sprocket wheel onto a larger one. In this case the chain must climb up the side wall of the larger sprocket wheel in order to come into engagement with the sprocket teeth. Further, every tooth end of the conventional arrangement lies slightly back from either of the side walls of the sprocket wheel body, that is, there exists a constant small distance between every tooth end and the side wall surface of the sprocket wheel body. This small distance occasionally adversely affects the chain catches when it the sprocket teeth, since the flexibility of the chain is quite limited. Thus, the disadvantages in conventional sprocket wheels are patently obvious.
The present invention contemplates an improvement over known bicycle sprocket wheels, in which the foregoing and other disadvantages are substantially overcome. According to the present invention, the teeth of the sprocket wheel are bilaterally zigzaged like those of saw blades, whereby the chain can be shifted more quickly and smoothly even from "top" to "low", that is, from the smallest sprocket wheel to the largest one.
It is, therefore, the principal object of the present invention to provide an improved sprocket wheel for a multi-speed free wheel assembly for a bicycle, wherein the teeth of the sprocket wheel are alternately set, right and left, for achieving a more efficient gear change. Further objects and novel features of the present invention will become more apparent from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the present invention.